Durations don't stack, IIRC... it's just the damage part. I'm pretty sure of that, anyway.

But like another poster said before me, stacking poisons pretty much requires you to custom-make them so that they have multiple effects that make them slightly different from one another... this way, you have (for example) 5 different kinds of poisons that create the same weakness and magnify it one right after the other. There is also no limit on how vulnerable you can make an enemy... the only limit is how many different kinds of poisons you can apply before the duration runs out. If you have a 100 alchemy and the right perks, you can achieve some astonishing damage after using weakness poisons -- easily one-shotting Alduin on master difficulty kind of damage.

If you use +alchemy gear that is self-enchanted to increase the alchemy skill, you can get well over a 100 alchemy when you create your poisons, and just make them sick and OP if you want. The game mechanics are there if you want to use them.
-Loth
Edit: I'm not sure you can put more than one poison on an arrow... I'm pretty sure you have to shoot multiple arrows to stack poisons on a target.